In order to expedite cash flow and to minimise the period between development drilling and production flow, more and more companies operating in the oil and gas business are resorting to what is commonly referred to as `Early Production Systems`.
These `Early Production Systems` use a method of predrilling wells prior to the installation of jacket structures which allows an operator to mate a completed production jacket over pre-drilled wells which are subsequently tied back to the surface and can be brought into production within a short period of completing the topside of the production jacket.
The drilling components used to pre-drill wells have been developed to provide such features as needed for effective reconnection of casing strings which were disconnected prior to installation of the jacket. These systems, commonly referred to as `mudline casing support equipment for jack up operations` and `subsea wellhead equipment for floating rig operations` are organized in a fixed grid structure over which the production jacket is placed so that the tie-back strings, guided through fixed guides which are part of the platform structure, can enter connection receptacles which are part of the mudline support system or the subsea wellhead system. Once the casing strings are tied-back, they are terminated on the production deck of the platform with the use of conventional surface wellhead equipment.
It is desirable that the tied-back casing strings should be under tension on installation, because heat generated by production fluids within the production tubing causes linear expansion of the casings which could otherwise cause them to buckle through induced compression. The casing strings therefore are tensioned at the surface wellhead and wedges are driven in between the casings and the high-pressure wellhead housing to maintain the tension. However, this known wedging system is imprecise in the amount of tension maintained as slippage can occur as the wedges are driven, and this becomes an acute problem on relatively short lengths of casing.
During drilling operations also it is often necessary to attach a length of casing at its lower end and connect its upper end to a fixture at a wellhead, in which case the upper end requires a fixing system allowing precise connection of the upper end. A similar wedging system to that used in tie-back operations can be used, with the same disadvantages, or the upper end of the casing may be cut to the desired length and a "slip-on" wellhead used. There is some doubt as to whether current designs of slip-on wellheads provide a secure fixing of the wellhead to the casing in a manner capable of withstanding very high fluid pressures such as those experienced during well blow-out.
In such operations it may or may not be necessary to pretension the casing before fixing; if the casing is not likely to be subject to substantial temperature changes pretensioning can be dispensed with.